The present technology relates to an image forming apparatus including a developing unit that supplies developer so as to develop an electrostatic latent image based on image information. This technology also relates to an image forming method including the step of supplying developer from a developing unit so as to develop an electrostatic latent image based on image information.
Some electrophotographically image-forming apparatus such as copying machines and printers use two-component developer, which consists of carrier and toner.
An image forming apparatus using two-component developer includes a photoreceptor drum and a developing unit, which includes a developing sleeve fitted with a magnet in it. The developing unit stirs and mixes the carrier and toner of the developer in it, so that the toner is charged electrostatically. The charged toner sticks to the particles of the carrier, which are then attracted magnetically to the circumferential surface of the developing sleeve. A magnetic brush consisting of carrier and toner is formed on the sleeve surface. The rotation of the developing sleeve moves the carrier and the toner out of the developing unit to the position where the magnetic brush is close to or in contact with the circumferential surface of the photoreceptor drum on which an electrostatic latent image has been formed. This causes the toner in the magnetic brush to be attracted electrostatically to the latent image, so that a toner image is developed on the drum surface.
When the apparatus forms an image with a high coverage rate consecutively on sheets of paper, the apparatus consumes a large amount of toner, so that toner needs to be supplied to the developing unit. Immediately after toner is supplied to the developing unit, the toner is not yet charged electrically. This makes it necessary to stir the supplied toner before the toner is supplied to the photoreceptor drum.
Therefore, the apparatus needs to interrupt its image forming operation so that toner can be supplied to the developing unit and stirred.
Immediately after toner is supplied to the developing unit, the toner and the carrier are stirred and mixed insufficiently. Accordingly, even though the image forming operation is restarted soon after toner is supplied to the developing unit and stirred, a large part of the toner is charged insufficiently, so that the formed image is low in quality. This makes a difference in image quality between the sheets on which the image is formed before and after the image forming operation is interrupted.
In particular, because recent image forming apparatus form images at high speed, the foregoing problem arises remarkably. There are demands for image forming apparatus that can keep forming images at high speed without lowering the quality of the images and interrupting the image forming operation.
For example, JP-H07-121018A discloses a recent image forming apparatus, which includes a developing unit fitted with a sensor for sensing the toner concentration in the unit. Until the sensed concentration reaches a predetermined value after toner starts to be supplied to the developing unit, the amount of toner supplied per unit time to the unit is smaller than when the apparatus is in a steady state. This prevents the amount of toner in the developing unit from increasing in comparison with the amount of carrier in this unit suddenly after toner starts to be supplied to the unit. The sudden increase would vary the toner concentration in the two-component developer.
As stated above, until the toner concentration reaches the predetermined value after toner starts to be supplied to the developing unit, the amount of toner supplied per unit time to the unit is smaller than when the apparatus is in its steady state. As a result, when the apparatus forms an image with a high coverage rate consecutively on sheets of paper, the apparatus needs to interrupt its image forming operation so that toner can be supplied in time to the developing unit. The smaller amount of toner supplied to the developing unit makes the interrupting time longer, so that it is impossible to form the image at a higher speed.
The object of the present technology is to provide an apparatus and a method that can form an image with a high coverage rate consecutively on sheets of paper at a high speed without lowering the quality of the image and interrupting the image forming operation of the apparatus.